When currents of sufficient amplitude are passed through brain tissue, the activity of neurons can be modulated. This is the basis of the most commonly used methods of stimulating the brain; however there has been little systematic quantitative study on how the current field couples to a neuron. The questions that I wish to examine are how neuronal geometry and orientation, with respect to a current field, influence the "capture" of current by a cell and how the type of field itself (steady, periodic; spatial distribution) affects the probability of neuronal modulation. In essence I want to know whether specific cells can be selectively modulated by the application of suitably chosen current fields. The methods are based on exploiting the well known geometrical organization of the cerebellar cortex and its neurons in three preparations: the in vivo rat, the in vitro turtle and the guinea pig brain slice. Currents will be a) steady b) periodic c) aperiodic. Neuronal responses will be measured by a variety of conventional techniques including intracellular recording, and extracellular unit, field potential and K+ measurement. The distribution of applied fields will be mapped along with the local impedance properties of the tissue. Concommitantly with the experimental studies, a theoretical analysis of the problem will be made using cable theory and appropriate field equations. A major objective of this work is to combine the experimental and theoretical aspects. The study involves the disciplines of neurophysiology and biophysics. The results of the project will relate to two areas; a) a better description of the biophysics of nerve tissue, b) improved information on how to selectively excite, or inhibit, specific nerve cells using appropriately designed current fields. These topics are directly related to he clinical use of electrical stimulation and to problems of the role of current flow in development and regeneration.